1. Field of the Invention
The present invention relates to a moving picture expert group (MPEG)-2 transport stream (TS) multiplexer for an asynchronous transfer mode (ATM) network adaptation and, more particularly, to a multiplexer which multiplexes MPEG-2 video and MPEG-1 audio signals for real time transmission and their ATM network adaptation.
2. Discussion of Related Art
For using the MPEG-2 audio/video (A/V) signals in a real time communication, a transport stream in the system layer (TS) is required. The system layer of the MPEG-2 can be output through two streams. First, a program stream (PS) is for the error-free situation, and has no restriction in length. Second, the TS is for the real time transmission. The length of TS is fixed to 188 bytes and has 4 byte header and 184 byte payload.
To retrieve the MPEG-2 A/V data in the receiver side, not only the data information but also the timing information has to be built in the program clock reference (PCR) packet and at least transmitted once per 0.1 second. Program map table (PMT) packet and program association table (PAT) packet are transmitted to denote the extra channel information. The input of the MPEG-2 TS multiplexer is originally formatted in the form of packetized elementary stream (PES). The length of the PES may be varied or fixed.
In case of the A/V PES, the data stream of 184 byte unit is input to the TS multiplexer, and 4 byte header is added to TS payload. Therefore, 188-byte A/V TS is generated and transmitted to the ATM adaptator. In case of the PCR packet, a timer formed in the TS multiplexer is used for the packet multiplexing. In case of PMT packet and PAT packet, table data are stored by a local processor during the initialization, the packets are transmitted to the ATM adaptator after being processed in the TS multiplexer by the timer.
An ATM cell of 53 octet is used for providing the services of fixed bit rate in B-ISDN. TS and PS provided by MPEG-2 system are used for receiving MPEG A/V data in B-ISDN. But, in the communication environment, only MPEG-2 TS can be used. This TS is composed of a 4-byte header and a 184-byte payload, so that it can communicate through an interface with a communication network. Originally, the TS is designed for ATM adaptation layer 1 (AAL 1). However, the TS is currently recommended to be used for AAL5. Accordingly, for adapting the 188-byte TS multiplexed with the MPEG A/V signals to the ATM network, an 8-byte AAL5 trailer is added to the two successive TSs (188.times.2). And in case of structuring this into 48-byte AAL5-PDU, the 8 ATM cells are made and transmitted to ATM network.
A structure of a protocol data unit (PDU) of the AAL5 protocol is formed in a manner that the PTI bit value of the ATM header is set for "0" in the first/middle PDU part, and set for "1" if the last trailer is included, through a payload type indication (PTI) of the ATM header. Therefore, the original TS boundary can be found out and retrieved at the receiving part.
Conventionally, in designing the MPEG-2 TS multiplexer, the number of the A/V packet is counted to transmit the PCR packet, but in this case the cell loss in the ATM network is not considered.